bhyve: avoid theoretical stack buffer overflow from integer overflow

Use the proper size_t type to match strlen's return type.  This is not
exploitable in practice as this parses command line arguments, which
are limited to well below 2^31 bytes.

This is a minimal change to address the reported issue; hda_parse_config
and the rest of this file will benefit from further review.

Reported by:	Fakhri Zulkifli
Reviewed by:	jhb, markj
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

usr.sbin/bhyve/pci_hda.c

@@ -324,15 +324,14 @@ hda_parse_config(const char *opts, const char *key, char *val)
 	char buf[64];
 	char *s = buf;
 	char *tmp = NULL;
-	int len;
+	size_t len;
 	int i;
 
 	if (!opts)
 		return (0);
 
 	len = strlen(opts);
-
-	if (len >= 64) {
+	if (len >= sizeof(buf)) {
 		DPRINTF("Opts too big\n");
 		return (0);
 	}